113 research outputs found
Physics of InAIAs/InGaAs Heterostructure Field-Effect Transistors
Contains an introduction, reports on three research projects, research conclusions and a list of publications.Joint Services Electronics Program Contract DAAHO4-95-1-003
Physics of Hemterostructure Field-Effect Transistors
Contains an introduction, report on one research project and a list of publications.Joint Services Electronics Program Contract DAAH04-95-1-0038Texas Instruments Agreement dated 11/23/9
Physics of InAIAs/InGaAs Heterostructure Field-Effect Transistors
Contains an introduction, a report on one research project and a list of publications and conference papers.Joint Services Electronics Program Contract DAAH04-95-1-0038Texas Instrument
Physics of InAIAs/InGaAs Heterostructure Field-Effect Transistors
Contains an introduction, reports on two research projects and a list of publications.Joint Services Electronics Program Contract DAAL03-92-C-0001Joint Services Electronics Program Grant DAAH04-95-1-0038Raytheon Corporation Contract 90-58203Texas Instruments Agreement dated 08/14/9
The Large-scale and Small-scale Clustering of Lyman-Break Galaxies at 3.5 < z< 5.5 from the GOODS survey
We report on the angular correlation function of Lyman-break galaxies (LBGs)
at z~4 and 5 from deep samples obtained from the Great Observatories Deep
Origins Survey (GOODS). Similar to LBGs at z~3, the shape of w(theta) of the
GOODS LBGs is well approximated by a power-law with slope beta~0.6 at angular
separation theta > 10 arcsec. The clustering strength of z~4, 5 LBGs also
depends on the rest-frame UV luminosity, with brighter galaxies more strongly
clustered than fainter ones, implying a general correlation between halos' mass
and LBGs' star-formation rate. At smaller separations, w(theta) of deep samples
significantly exceeds the extrapolation of the large-scale power-law fit,
implying enhanced spatial clustering at scales r < 1 Mpc. We also find that
bright LBGs statistically have more faint companions on scales theta < 20
arcsec than fainter ones, showing that the enhanced small-scale clustering is
very likely due to sub-structure, namely the fact that massive halos can host
multiple galaxies. A simple model for the halo occupation distribution and the
CDM halo mass function reproduce well the observed w(theta). The scaling
relationship of the clustering strength with volume density and with redshift
is quantitatively consistent with that of CDM halos. A comparison of the
clustering strength of three samples of equal luminosity limit at z ~ 3, 4 and
5 shows that the LBGs at z~5 are hosted in halos about one order of magnitude
less massive than those in the lower redshift bins, suggesting that
star-formation was more efficient at higher-redshift.Comment: replaced with the version accepted for publication in ApJ. 46 pages,
10 figures; minor changes to text, one subsection adde
Mapping the Dark Matter From UV Light at High Redshift: An Empirical Approach to Understand Galaxy Statistics
We present a simple formalism to interpret two galaxy statistics, the UV
luminosity function and two-point correlation functions for star-forming
galaxies at z~4, 5, 6 in the context of LCDM cosmology. Both statistics are the
result of how star formation takes place in DM halos, and thus are used to
constrain how UV light depends on halo properties such as mass. The two
measures were taken from the GOODS data, thus ideal for joint analysis. The two
physical quantities we explore are the SF duty cycle, and the range of L_UV
that a halo of mass M can have (mean and variance). The former addresses the
typical duration of SF activity in halos while the latter addresses the
averaged SF history and regularity of gas inflow into these systems. We explore
various physical models consistent with data, and find the following: 1) the
typical duration of SF observed in the data is <0.4 Gyr (1 sig), 2) the
inferred scaling law between L_UV and halo mass M from the observed slope of
the LFs is roughly linear at all redshifts, and 3) L_UV for a fixed halo mass
decreases with time, implying that the SF efficiency (after dust extinction) is
higher at earlier times. We explore several physical scenarios relating star
formation to halo mass, but find that these scenarios are indistinguishable due
to the limited range of halo mass probed by our data. In order to discriminate
between different scenarios, we discuss constraining the bright-faint galaxy
cross-correlation functions and luminosity-dependence of galaxy bias.
(Abridged)Comment: 24 pages, 16 figures: matches published version -- Astrophysical
Journal 695 (2009) 368-39
Galaxies in a Simulated CDM Universe II: Observable Properties and Constraints on Feedback
We compare the properties of galaxies that form in a cosmological simulation
without strong feedback to observations at z=0. We confirm previous findings
that models without strong feedback overproduce the observed galaxy baryonic
mass function, especially at the low and high mass extremes. Through
post-processing we investigate what kinds of feedback would be required to
reproduce observed galaxy masses and star formation rates. To mimic an extreme
form of "preventive" feedback (e.g., AGN radio mode) we remove all baryonic
mass that was originally accreted via "hot mode" from shock-heated gas. This
does not bring the high mass end of the galaxy mass function into agreement
with observations because much of the stellar mass in these systems formed at
high redshift from baryons that originally accreted via "cold mode" onto lower
mass progenitors. An efficient "ejective" feedback mechanism, such as supernova
driven winds, must reduce the masses of these progenitors. Feedback must also
reduce the masses of lower mass z=0 galaxies, which assemble at lower redshifts
and have much lower star formation rates. If we monotonically re-map galaxy
masses to reproduce the observed mass function, but retain the simulation's
predicted star formation rates, we obtain fairly good agreement with the
observed sequence of star-forming galaxies but fail to recover the observed
population of passive, low star formation rate galaxies. Suppressing all hot
mode accretion improves agreement for high mass galaxies but worsens the
agreement at intermediate masses. Reproducing these z=0 observations requires a
feedback mechanism that dramatically suppresses star formation in a fraction of
galaxies, increasing with mass, while leaving star formation rates of other
galaxies essentially unchanged.Comment: MNRAS in press. 15 pages, 5 figures, minimal changes from the first
versio
The Relation Between SFR and Stellar Mass for Galaxies at 3.5 6.5 in CANDELS
Distant star-forming galaxies show a correlation between their star formation
rates (SFR) and stellar masses, and this has deep implications for galaxy
formation. Here, we present a study on the evolution of the slope and scatter
of the SFR-stellar mass relation for galaxies at using
multi-wavelength photometry in GOODS-S from the Cosmic Assembly Near-infrared
Deep Extragalactic Legacy Survey (CANDELS) and Spitzer Extended Deep Survey. We
describe an updated, Bayesian spectral-energy distribution fitting method that
incorporates effects of nebular line emission, star formation histories that
are constant or rising with time, and different dust attenuation prescriptions
(starburst and Small Magellanic Cloud). From =6.5 to =3.5 star-forming
galaxies in CANDELS follow a nearly unevolving correlation between stellar mass
and SFR that follows SFR with at and at . This evolution requires a star formation
history that increases with decreasing redshift (on average, the SFRs of
individual galaxies rise with time). The observed scatter in the SFR-stellar
mass relation is tight, yr 0.4 dex, for galaxies with dex.
Assuming that the SFR is tied to the net gas inflow rate (SFR
), then the scatter in the gas inflow rate is also
smaller than 0.30.4 dex for star-forming galaxies in these stellar mass and
redshift ranges, at least when averaged over the timescale of star formation.
We further show that the implied star formation history of objects selected on
the basis of their co-moving number densities is consistent with the evolution
in the SFR-stellar mass relation.Comment: 31 pages, 24 figures, accepted for publication in Ap
A Critical Assessment of Stellar Mass Measurement Methods
In this paper we perform a comprehensive study of the main sources of random
and systematic errors in stellar mass measurement for galaxies using their
Spectral Energy Distributions (SEDs). We use mock galaxy catalogs with
simulated multi-waveband photometry (from U-band to mid-infrared) and known
redshift, stellar mass, age and extinction for individual galaxies. Given
different parameters affecting stellar mass measurement (photometric S/N
ratios, SED fitting errors, systematic effects, the inherent degeneracies and
correlated errors), we formulated different simulated galaxy catalogs to
quantify these effects individually. We studied the sensitivity of stellar mass
estimates to the codes/methods used, population synthesis models, star
formation histories, nebular emission line contributions, photometric
uncertainties, extinction and age. For each simulated galaxy, the difference
between the input stellar masses and those estimated using different simulation
catalogs, , was calculated and used to identify the most
fundamental parameters affecting stellar masses. We measured different
components of the error budget, with the results listed as follows: (1). no
significant bias was found among different codes/methods, with all having
comparable scatter; (2). A source of error is found to be due to photometric
uncertainties and low resolution in age and extinction grids; (3). The median
of stellar masses among different methods provides a stable measure of the mass
associated with any given galaxy; (4). The deviations in stellar mass strongly
correlate with those in age, with a weaker correlation with extinction; (5).
the scatter in the stellar masses due to free parameters are quantified, with
the sensitivity of the stellar mass to both the population synthesis codes and
inclusion of nebular emission lines studied.Comment: 33 pages, 20 Figures, Accepted for publication in Astrophysical
Journa
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